The aim of this research is to develop and perfect electrodes which will give a continuous measurement of lactate concentration in whole blood, tissue homogenates, on organ surfaces, and in cells. The electrodes are based upon measurement of peroxide-generated platinum anodic currents at a constant applied voltage. The currents are in the nanoampere range for the tissue and blood electrodes and in the picoampere range, or possibly below, for the microelectrodes. This lactate electroenzymatic system is based upon the specific stoichiometric conversion of lactate to pyruvate in the presence of oxygen with the formation of hydrogen peroxide. Because the electrodes will be designed by combinations of membranes and immobilized enzyme to be lactate-diffusion-limited it will be possible to use them to continuously record lactate levels. The electrodes will be placed at Swan-Ganz catheter tips and tested by using them in mixed venous blood in anesthetized dogs. Hypovolemia and sodium lactate infusion will be used to induce increases in lactate. Samples will be analyzed for comparison. Other electrodes will be designed for use for surface measurements on the liver and other organs in cats and dogs. These electrodes will be modifiable, if necessary, for fetal scalp blood measurements. Microelectrodes will be drawn glass and filled with an electrically conductive mixture such that the enzyme lactate oxidase can be added to form an electrochemically functional interface. Special attention will be given to designing electrodes which can be stored without loss of activity and which will function in biological fluids such as blood and tissue, as well as in pure buffers. Such lactate sensors will be valuable in clinical cardiology, critical care medicine, basic research in physiology, obstetrics and gynecology, neonatal care, and in diabetes research and management of diabetic patients.